Underwater pipelines comprise a number of pipes joined to total lengths of hundreds of kilometers. The pipes are of normally 12-metre unit length, and relatively large diameters ranging between 0.2 and 1.5 metres, and each comprise a steel cylinder; a first coating of polymer material to protect the steel pipe; and possibly a second coating of Gunite or cement to weigh down the pipe. In some applications, the pipes and underwater pipelines do not need and therefore have no second coating.
To weld the steel cylinders to one another, the end portions of each pipe have no first or second coating. The pipes are joined at on-land installations into multiple-unit-length pipes, as well as on pipeline-laying vessels, on which unit-length or multiple-unit-length pipes are joined to others already joined to other pipes to form part of the underwater pipeline.
The actual joining operation comprises welding the steel cylinders, normally in a number of weld passes, and forming the protective joint coating, and possibly also a weighting joint coating. Once the annular weld bead is formed between two steel cylinders, the cutback extends astride the weld, along a portion with no first or second coating. In other words, the cutback is defined substantially by the end portions of the pipes, extends axially between two end portions of the first coating, and must be coated with the protective joint coating to prevent corrosion.
Applying the protective joint coating to the cutback is known as “field joint coating”, and comprises coating the cutback with three coats (in some applications, the protective joint coating comprises two coats) to ensure protection and adhesion of the coats to the steel cylinders.
Applying the protective joint coating to the cutback comprises shot blasting to roughen the outer surface of the cutback; heating, e.g. induction heating, the cutback to 250° C.; spraying the cutback with powdered epoxy (FBE—Fusion Bonded Epoxy) resin, which, in contact with the cutback, forms a relatively thin first coat or “primer”; spraying the cutback, on top of the first coat, with a modified copolymer, which acts as adhesive and, in contact with the first coat, forms a relatively thin second coat; applying a third so-called “top coat”, which also extends partly over the first coating; and then possibly applying the weighting joint coating.
Welding, non-destructive weld testing, shot blasting, cutback heating, and protective and weighting joint coating are performed at joining stations equally spaced along the path of the pipes (or of the partly formed pipeline, when the pipes are joined to this) on a firing line. The pipes and the partly formed pipeline are therefore fed in steps through the joining stations, and are stopped at each joining station for a length of time determined by the longest operation, which, at present, is shot blasting the cutback.
Shot blasting provides for effectively roughening the outer surface of the cutback, but, in addition to being relatively time-consuming, also has other drawbacks: it calls for the use of large-size shot conveying, recovery, and filtering equipment; and it produces large amounts of dust, which contaminate the workplace and call for the installation of additional dust-extraction equipment.
All these drawbacks are further compounded by the firing line being housed in a tunnel on the vessel, which means very little space is available in which to install the necessary equipment, and the tunnel is quickly saturated with dust.